Integrated maintenance bypass switch and method of operation thereof

ABSTRACT

In a backup power system capable of deriving power from alternate AC primary and DC backup power sources to provide continuous AC power to an AC load and including an inverter/switch module having DC and AC inputs and an AC output, a maintenance bypass switch for, and method of decoupling the AC primary and DC backup power sources from the backup power module to allow maintenance to be performed thereon. The maintenance bypass switch includes a selector switch configured to function in alternate normal, standby bypass and maintenance bypass modes of operation, wherein: (a) in transitioning from the normal mode to the standby bypass mode, the selector switch decouples an inverter in the inverter/switch module from the DC backup power source, (b) in transitioning from the standby bypass mode to the maintenance bypass mode, the selector switch further couples the AC primary power source directly to the AC load and (c) in the maintenance bypass mode, the selector switch decouples the AC primary power source from the AC input of the inverter/switch module, the DC backup power source from the DC input of the inverter/switch module and the AC output of the inverter/switch module to the AC load, the maintenance bypass switch providing the continuous AC power to the AC load throughout the transitioning.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to power systems and,more particularly, to a maintenance bypass switch that providessingle-switch isolation of power in a bypass mode.

BACKGROUND OF THE INVENTION

It is customary to have an alternate power source for an installation ifa primary, commercial power source is lost or becomes unreliable. Thealternate power source may take the form of, for example, a generatordriven by an internal combustion engine. When a transformation from thecommercial power source to the alternate source is required, a transferswitch is used to disconnect the commercial power source and thereafterto connect the alternate power source to the load. The transfer switchconventionally provides an open, or break-before-make, transitionbetween the two power sources. More specifically, the transfer switchdisconnects the initial commercial power source before connecting thealternate power source to the load. The open transition switching avoidsany potential surges that may occur as a result of the twounsynchronized sources applying power to the load simultaneously.

Open transition transfer switches to accommodate the precise applicationas described above are well known in the art. For instance, transferswitches utilizing a pair of circuit breakers and a mechanism foroperating the two circuit breakers in opposition are a product of formerpatents. U.S. Pat. No. 3,778,633 to DeVisser, et al., issued on Dec. 11,1973, entitled "Automatic Electric Power Source Transfer Apparatus,"discloses a device that includes two circuit breakers mounted end-to-endand operated in opposition by a ganging member driven rectilinearly by ascrew shaft engaging a traveling nut connected to the ganging member.Another transfer switch of this type uses two side-by-side circuitbreakers with handles. The handles are engaged by clevises attached to alever arm that is rotated about a pivot axis between the two circuitbreakers. In still another transfer switch using two side-by-sidecircuit breakers, a plurality of slides engaging the circuit breakerhandles are driven in opposition by gears having an eccentric pin toengage a camming surface on the slide.

The previously-described transfer switches provide open transitionoperation and thus momentarily interrupt power to the load. However, theopen transition switch, where there is absolute and unabridgedinterruption of power to the load for a period of time, is unacceptablein certain situations. For example, a digital computer will lose thecontents of its volatile memory if the power source is severed for evena very short interval. In yet another example, a central office withequipment serving a telephone network cannot be deprived of power foreven an instant, or service is lost throughout the network.

To accommodate such critical applications, uninterruptible powersupplies ("UPSs") have been developed to provide continuous power to thecomputer or central office equipment should the commercial power sourcebe lost. One illustration of an uninterruptible power supply is abattery that is ordinarily charged by a charging device connected to thecommercial power system. When commercial power source is lost, thebattery provides direct current ("DC") power to the load and an invertercoupled to and fed by the battery provides alternating current ("AC")power to the load. Conventionally, a special static switch, located inproximity to the inverter, provides a rapid open transfer between thecommercial power source and the inverter. The resulting interruption ofpower only lasts for a few milliseconds, thereby preserving theintegrity of the power to the load and not adversely affecting thedigital computer, central office equipment or other critical load.

Occasionally, the uninterruptible power supply, including the inverter,must be removed from the power system to perform routine maintenance,for example. In such cases, a bypassing mechanism should be providedthat is capable of disengaging the uninterruptible power supply from thepower system without provoking an interruption of the power to the load.Prior art bypass switches, such as U.S. Pat. No. 5,081,367 to Smith, etal., issued on Jan. 14, 1992, and entitled "Electric Power System withMaintenance Bypass for Uninterruptible Power Supply Using ClosedTransition Operation," disclose maintenance bypass switches fordisconnecting an interruptable power supply.

The maintenance bypass switch of Smith, et al. provides a first switchin series with the uninterruptible power supply and a second switch thatshunts the uninterruptible power supply and the first switch. The firstand second switches are circuit breakers having handles which togglebetween open and closed positions. The maintenance bypass switchincludes an operator that operates the two circuit breakers inopposition with a closed transition so that one switch is always closed.An interlock prevents operation of the maintenance bypass switch whenthe alternate power source of the uninterruptible power supply isselected by the static transfer switch. An isolation switch between theuninterruptible power supply and the commercial power source and alsoshunted by the second switch of the maintenance bypass switch is opentogether with the first switch of the maintenance bypass switch when theuninterruptible power supply is to be isolated for maintenance. However,the prior art bypass switches, including Smith, et al., are deficient insome significant respects.

First, the prior art bypass switches only provide single line switching;even in the bypass mode of operation, a current always flows through anunswitched neutral line. Therefore, a person servicing anuninterruptible power supply coupled to the prior art bypass switchesrisk injury if the neutral line carries an electrical current.

Second, the transfer and restore procedures of the prior art bypassswitches are cumbersome and unforgiving. To enter the bypass mode ofoperation, a user must meticulously follow the transfer sequenceinstructions. This involves simultaneously switching both themaintenance bypass switch (for AC bypass) and a separate inverterselector switch (for DC disconnect). If the user does not toggle theswitches precisely at the same time, the bypass mode will not beachieved successfully leading to a situation where the two sources maybe paralleled for an extended period of time or, worse yet, the loadwill decouple from the power source. Again, in the critical applicationsfor which the uninterruptible power supply is designed, loss of powerfor even an instant is unacceptable.

Third, while Smith, et al. addresses some of the aforementioneddeficiencies, Smith, et al. still does not provide a means for switchingthe DC source (i.e. battery) which may be hazardous depending on thevoltage and charge capacity of the battery. Finally, Smith, et al. doesnot provide AC isolation between the uninterruptible power supply andthe commercial power source in a single operation.

Accordingly, what is needed in the art is a maintenance bypass switchthat provides more reliable isolation of both AC and DC power andsimplifies entry into the bypass mode of operation by integrating theswitching of both AC and DC power into a single maintenance bypassswitch operable by a single selector.

SUMMARY OF TEE INVENTION

To address the above-discussed deficiencies of the prior art, it is aprimary object of the present invention to provide, in a backup powersystem capable of deriving power from alternate AC primary (oftencommercial) and DC backup (often battery) power sources to providecontinuous AC power to an AC load and including an inverter/switchmodule having DC and AC inputs and an AC output, a maintenance bypassswitch for, and method of decoupling the AC primary and DC backup powersources from the backup power module to allow maintenance to beperformed thereon.

The maintenance bypass switch includes a selector switch configured tofunction in alternate normal, standby bypass and maintenance bypassmodes of operation, wherein, in the normal mode, the selector switchcouples the AC primary power source to the AC input of theinverter/switch module, the DC backup power source to the DC input ofthe inverter/switch module and the AC output of the inverter/switchmodule to the AC load. In transitioning from the normal mode to thestandby bypass mode, the selector switch decouples an inverter in theinverter/switch module from the DC backup power source. In transitioningfrom the standby bypass mode to the maintenance bypass mode, theselector switch further couples the AC primary power source directly tothe AC load. In the maintenance bypass mode, the selector switchdecouples the AC primary power source from the AC input of theinverter/switch module, the DC backup power source from the DC input ofthe inverter/switch module and the AC output of the inverter/switchmodule to the AC load, the maintenance bypass switch providing thecontinuous AC power to the AC load throughout the transitioning.Therefore, the present invention introduces a maintenance bypass switchthat integrates AC and DC switch functions, allowing single-switchactivation of the maintenance bypass mode.

In a preferred embodiment of the present invention, the selector switchselectively couples and decouples both phase and neutral conductors ofthe AC primary and DC backup power sources. By also switching theneutral conductors, complete isolation of the module from the AC primaryand DC backup power sources is possible.

In a preferred embodiment of the present invention, the maintenancebypass switch further comprises a status indicator coupled to theselector switch for indicating the mode of the selector switch. Althoughnot necessary, the status indicator preferably presents informationconcerning the mode of the selector switch to a user. The statusindicator may be near or remote from the selector switch.

In a preferred embodiment of the present invention, the maintenancebypass switch further comprises an alarm circuit coupled to the selectorswitch for transmitting a signal to an external alarm when the ACprimary and DC backup power sources are disconnected from the AC load.Although not necessary, the alarm circuit allows a warning signal to begiven to a user via the external alarm. The external alarm may be nearor remote from the selector switch.

In a preferred embodiment of the present invention, the inverter/switchmodule comprises a static transfer switch. Those of ordinary skill inthe pertinent art realize that static transfer switches accomplish fastsource switching and are thus commonly employed in uninterruptible powersupplies.

In a preferred embodiment of the present invention, the selector switchcomprises a rotary cam switch. Alternatively, the selector switch maycontain one or more interlocked switches.

In a preferred embodiment of the present invention, the selector switchcauses a breaker within the inverter/switch module to trip, therebydecoupling the inverter from the DC backup power source. In a related,preferred embodiment of the present invention, in transitioning theselector switch from the maintenance bypass mode to the normal mode, theDC breaker remains tripped, the DC breaker capable of being resetseparately to recouple the inverter to the DC backup power source. TheDC breaker is often included as a portion of the inverter/switch moduleand affords additional protection to the inverter.

The foregoing has outlined, rather broadly, preferred and alternativefeatures of the present invention so that those skilled in the art maybetter understand the detailed description of the invention thatfollows. Additional features of the invention will be describedhereinafter that form the subject of the claims of the invention. Thoseskilled in the art should appreciate that they can readily use thedisclosed conception and specific embodiment as a basis for designing ormodifying other structures for carrying out the same purposes of thepresent invention. Those skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention in its broadest form.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a block diagram of a power system employing a priorart maintenance bypass switch;

FIG. 2 illustrates a block diagram of a power system employing anembodiment of a maintenance bypass switch according to the principles ofthe present invention;

FIG. 3 illustrates a front elevational view of the chassis enclosing themaintenance bypass switch of FIG. 2; and

FIG. 4 illustrates a schematic diagram of an alternative embodiment of amaintenance bypass switch according to the principles of the presentinvention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a block diagram of a powersystem 100 employing a prior art maintenance bypass switch 110. Themaintenance bypass switch 110 contains a first, second and third circuitbreaker or switches 120, 130, 140, respectively. The power system 100also includes an inverter 150 having a DC/AC converter 160, a statictransfer switch ("STS") 170 and a circuit breaker 175. The DC/ACconverter 160 converts direct current to alternating current and the STS170 provides fast source switching in the inverter 150. An AC primarypower source 180 is connected through the maintenance bypass switch 110to the inverter 150 and a load 190. A DC backup power source 185 isconnected through the inverter 150 and, after transformation to AC,through the maintenance bypass switch 110 to the load 190.

In the illustrated embodiment, the maintenance bypass switch 110 ispresented with the first and second circuit breakers 120, 130 closed andthe third circuit breaker 140 open. The AC primary power source 180,therefore, traverses both the maintenance bypass switch 110 (through thefirst and second circuit breakers 120, 130) and the inverter 150(through the STS 170) en route to the load 190. The DC backup powersource 185 traverses the inverter 150 (through the circuit breaker 175,the DC/AC converter 160 and the STS 170) and, after transformation toAC, traverses the maintenance bypass switch 110 (through the firstcircuit breaker 120) en route to the load 190.

To effectively bypass the inverter 150, the following procedures must bestrictly adhered to. First, operate a switch on a panel (not shown) ofthe inverter 150 to a line position to connect the AC primary powersource 180 to the load 190 through the second circuit breaker 130, theSTS 170 and the first circuit breaker 120, consecutively. Second, closethe third circuit breaker 140 through a lever on a panel (not shown) ofthe maintenance bypass switch 110 to provide a direct connection for theAC primary power source 180 to the load 190. Third, close the firstcircuit breaker 120 through a lever on the panel of the maintenancebypass switch 110 to disconnect the path established in the first stepabove. Fourth, disengage the circuit breaker 175 by physically openingthe circuit breaker 175 inside the chassis (not shown) of the inverter150 to disconnect the DC backup power source 185 from the inverter 150.Finally, open the second circuit breaker 130 through a lever on thepanel of the maintenance bypass switch 110 to disconnect the AC primarypower source 180 from the inverter 150. Thus, in the bypass mode ofoperation, the second circuit breaker 130 is open and the third circuitbreaker 140 is closed providing a direct path for the AC primary powersource 180 to the load 190 thereby bypassing the inverter 150.

To restore the inverter 150 operation the procedure is equally ascumbersome. First, close the circuit breaker 175 inside the inverter 150to apply the DC backup power source 185 to the inverter 150. Second,close the second circuit breaker 130 through the lever on the panel ofthe maintenance bypass switch 110 to connect the AC primary power source180 to the inverter 150. Third, rotate the switch on the panel of theinverter 150 to a line position. Fourth, close the first circuit breaker120 through the lever on the panel of the maintenance bypass switch 110to connect the inverter 150 to the load 190. Finally, open the thirdcircuit breaker 140 through the lever on the panel of the maintenancebypass switch 110 to break the direct connection between the AC primarypower source 180 and the load 190.

As previously discussed, the prior art maintenance bypass switch 110endures several disadvantages. First, the transfer (or bypass) andrestore procedures are cumbersome, confusing and unforgiving. To enterthe bypass mode of operation, a user must meticulously follow thetransfer sequence instructions. Moreover, the bypass mode of operationis potentially unsafe for maintenance. The prior art maintenance bypassswitch 110 only provides single line switching; even in the bypass modeof operation, a current always flows through an unswitched neutral line.Therefore, a person servicing the inverter 150 coupled to themaintenance bypass switch 110 risks injury if the neutral line carriesan electrical current. Also, the prior art maintenance bypass switch 110presents additional risks by requiring separate AC primary power source180 connect and disconnect switching.

Turning now to FIG. 2, illustrated is a block diagram of a power system200 employing an embodiment of a maintenance bypass switch 210 accordingto the principles of the present invention. The maintenance bypassswitch 210 includes a selector switch 220, a status indicator circuit223, an alarm circuit 225 and a DC breaker 230. While the selectorswitch 220, in the illustrated embodiment, comprises a multi-connectionrotary cam switch, alternatively, the selector switch may contain one ormore interlocked conventional switches. The power system 200 alsoincludes an inverter/switch module 240 (i.e. backup power module) havinga DC/AC converter 250, a static transfer switch ("STS") 260 and acircuit breaker (an inverter/switch module DC breaker) 270. The DC/ACconverter 250 converts direct current to alternating current and the STS260 provides fast source switching in the inverter/switch module 240. AnAC primary power source 280 is connected through the maintenance bypassswitch 210 to the inverter/switch module 240 and a load (i.e. an ACload) 290. A DC backup power source 285 is also connected through themaintenance bypass switch 210 to the inverter/switch module 240 and,after transformation to AC, to the load 290.

The power system 200 derives power from the alternate AC primary powersource 280 (often commercial) and the DC backup power source 285 (oftenbattery) to provide continuous AC power to the load 290 and theinverter/switch module 240 having DC and AC inputs 243, 246 and an ACoutput 249. The maintenance bypass switch 210 decouples the AC primarypower source 280 and the DC backup power source 285 from theinverter/switch module 240 (or other backup power module) to allowmaintenance to be performed thereon. The maintenance bypass switch 210is, therefore, capable of decoupling single- or multi-phase powersources from the inverter/switch module 240.

The selector switch 220 of the maintenance bypass switch 210 functionsin one of several modes including an alternate normal ("NORM"), astandby bypass ("SBP") and a maintenance bypass ("MBP") mode ofoperation. In the normal mode, the selector switch 220 couples the ACprimary power source 280 to the AC input 246 of the inverter/switchmodule 240, the DC backup power source 285 to the DC input 243 of theinverter/switch module 240 and the AC output 249 of the inverter/switchmodule 240 to the load 290. In transitioning from the normal mode to thestandby bypass mode, the selector switch 220 decouples theinverter/switch module 240 from the DC backup power source 285. Intransitioning from the standby bypass mode to the maintenance bypassmode, the selector switch 220 further couples the AC primary powersource 280 directly to the load 290. In the maintenance bypass mode, theselector switch 220 decouples the AC primary power source 280 from theAC input 246 of the inverter/switch module 240, the DC backup powersource 285 from the DC input 243 of the inverter/switch module 240 andthe AC output 249 of the inverter/switch module 240 to the load 290. Themaintenance bypass switch 210 provides continuous AC power to the load290 throughout the transitioning. Therefore, the present inventionintroduces a maintenance bypass switch 220 that integrates AC and DCswitch functions, allowing single-switch activation of the maintenancebypass mode.

Additionally, in transitioning from the maintenance bypass mode to thenormal mode, the DC breaker 230 remains tripped. The DC breaker 230 iscapable of being reset separately to recouple the inverter/switch module240 to the DC backup power source 285. The DC breaker 230 may also beincluded as a portion of the inverter/switch module 240 (as discussedbelow) and affords additional protection to the inverter/switch module240.

The selector switch 220 also selectively couples and decouples bothphase and neutral conductors of the AC primary and DC backup powersources 280, 285. Switching the neutral conductors provides completeisolation of the inverter/switch module 240 from the AC primary and DCbackup power sources 280, 285. The selector switch 220 still furtherinduces the circuit breaker 270 within the inverter/switch module 240 totrip, thereby decoupling the inverter/switch module 240 from the DCbackup power source 285.

The status indicator circuit 223 is coupled to the selector switch 220to monitor the operational status of the maintenance bypass switch 210.The status indicator circuit 223 is connected to a status indicator (seedescription with respect to FIG. 3) to indicate the operational statusof the maintenance bypass switch 210 to a user. The alarm circuit 225 iscoupled to the selector switch 220 for transmitting a signal to anexternal alarm 227 when the AC primary and DC backup power sources 280,285 are disconnected from the load 290. Although not necessary, thealarm circuit 225 allows a warning signal to be given to a user via theexternal alarm 227. The external alarm 227 may be near or remote fromthe selector switch 220.

The maintenance bypass switch 220 therefore provides several advantagesover the prior art maintenance bypass switches 120. First, there isvirtually no transfer and restore procedures to follow. To effectuatethe bypass mode of operation, rotate the selector switch 220 to amaintenance bypass position (see description with respect to FIG. 3). Torestore normal operation, rotate the selector switch 220 to a normalposition (see description with respect to FIG. 3) and close the DCbreaker 230. The transfer and restore procedures are uncomplicated andeffortless. Second, the bypass mode of operation is mistake proof withvirtually no chance of dropping the load 290 during transfer or restoreprocedures. Finally, the maintenance bypass switch 220 of the presentinvention is safe. The maintenance bypass switch 220 provides two lineswitching thereby totally de-energizing the inverter/switch module 240in the bypass mode of operation; the AC primary power source 280 is,also, connected and disconnected in a single switch.

Turning now to FIG. 3, illustrated is a front elevational view of thechassis 300 enclosing the maintenance bypass switch 210 of FIG. 2. Thechassis includes a front panel 310 having a selector guide 320 for theselector switch 220, a selector button 330 for the DC breaker 230, astatus indicator 340 and an operating instruction field 350. Theselector guide 320 includes a directional arm 322 to select one of theoperational modes of the maintenance bypass switch 210 including thenormal, standby bypass and maintenance bypass modes of operation. Theoperation of the maintenance bypass switch 210 in each of theoperational modes is discussed in more detail with respect to FIG. 2.The selector button 330 provides the mechanism to turn the DC breaker230 on or off and/or to indicate whether the DC breaker 220 is tripped.The status indicator 340 is coupled to the status indicator circuit 223to display the operational mode of the maintenance bypass switch 210 toa user. The status indicator 340 may be near (as illustrated in FIG. 3)or remote from the selector switch 220. Finally, the operatinginstruction field 350 provides an easily accessible location to describethe operating instructions of the maintenance bypass switch 220 to theuser.

Turning now to FIG. 4, illustrated is a schematic diagram of analternative embodiment of a maintenance bypass switch 400 according tothe principles of the present invention. Analogous to the maintenancebypass switch 210 of FIG. 2, the maintenance bypass switch 400 includesa selector switch 410, a status indicator circuit comprising a pluralityof individual status circuits 422, 424, 426, 428, an alarm circuitcomprising a relay 440 and a double-pole DC breaker 450 having ashunt-tree circuit 455. While the selector switch 410, in theillustrated embodiment, comprises a multi-connection rotary cam switch,alternatively, the selector switch may contain one or more interlockedconventional switches. An AC primary power source (not shown) isconnected through the maintenance bypass switch 400, at contacts 460, toan inverter/switch module (not shown), at contacts 465, and a load (notshown), at contacts 470. A DC backup power source (not shown) is alsoconnected through the maintenance bypass switch 400, at contacts 475, tothe inverter/switch module, at contacts 480, and, after transformationto AC, to the load, at contacts 470. The individual status circuits 422,424, 426, 428 provide a signal to a status indicator (not shown) toindicate the operational mode of the maintenance bypass switch 400. Therelay 440 is energized when the maintenance bypass switch 400 is in themaintenance bypass mode of operation and a signal may be transmittedthrough contacts 485 to an external alarm (not shown) to warn a user, ifnecessary.

The operation of the maintenance bypass switch 400 is analogous to theoperation of the maintenance bypass switch 210 as described with respectto FIG. 2. The following table (i.e. Table I) defines the operation ofthe selector switch 410 in the normal, standby bypass and maintenancebypass modes of operation.

                  TABLE I                                                         ______________________________________                                        SELECTOR CONTACT PAIRS                                                        SWITCH                                11-  1*-  3*-                           POSITION 1-2    3-4    5-6  7-8  9-10 12   2*   4*                            ______________________________________                                        NORM     C      0      C    0    C    C    0.sub.X                                                                            0                                      --     --     --   --                                                SBP      C.sub.Y                                                                              0      C    0    C    C    0.sub.X                                                                            0                             MBP      0.sub.Y                                                                              C      0    C    0    0    0    C                                      --     --     --   --                                                ______________________________________                                    

In Table I, the subscript _(x) denotes a momentary closure and thesubscript _(y) denotes a make before break contact.

In the illustrated embodiment, the selector switch 410 demonstrates thenormal mode of operation. In the normal mode, contact pairs 1-2, 5-6,9-10 and 11-12 are closed and contact pairs 3-4, 7-8, 1*-2* and 3*-4*are open. The selector switch 410 therein couples the AC primary powersource to the inverter/switch module, the DC backup power source to theinverter/switch module and the inverter/switch module to the load.During the transition from the normal mode to the standby bypass mode,the contact pairs as described with respect to the normal mode ofoperation remain unchanged except contact pairs 1*-2* are momentarilyclosed then opened and the DC breaker 450 is tripped (when the ACprimary power source is functioning). In the standby bypass mode ofoperation, the selector switch 410 decouples the inverter/switch modulefrom the DC backup power source. Finally, in the maintenance bypass modeof operation, contact pairs 1-2, 5-6, 9-10, 11-12 and 1*-2* are open andthe contact pairs 3-4, 7-8 and 3*-4* are closed. However, during thetransition to the maintenance bypass mode of operation, all contactpairs are closed before the aforementioned contact pairs are open. Inthe maintenance bypass mode, the selector switch 410 decouples the ACprimary power source from the inverter/switch module, the DC backuppower source from the inverter/switch module and the inverter/switchmodule to the load.

Although the present invention has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

What is claimed is:
 1. In a backup power system capable of derivingpower from alternate AC primary and DC backup power sources to providecontinuous AC power to an AC load and including an inverter/switchmodule having DC and AC inputs and an AC output, a maintenance bypassswitch for decoupling said AC primary and DC backup power sources fromsaid backup power system to allow maintenance to be performed thereon,comprising:a selector switch configured to function in alternate normal,standby bypass and maintenance bypass modes of operation, wherein:insaid normal mode, said selector switch couples said AC primary powersource to said AC input of said inverter/switch module, said DC backuppower source to said DC input of said inverter/switch module and said ACoutput of said inverter/switch module to said AC load, in transitioningfrom said normal mode to said standby bypass mode, said selector switchdecouples an inverter in said inverter/switch module from said DC backuppower source when said AC primary power source is functioning, intransitioning from said standby bypass mode to said maintenance bypassmode, said selector switch further couples said AC primary power sourcedirectly to said AC load, and in said maintenance bypass mode, saidselector switch decouples said AC primary power source from said ACinput of said inverter/switch module, said DC backup power source fromsaid DC input of said inverter/switch module and said AC output of saidinverter/switch module to said AC load, said maintenance bypass switchproviding said continuous AC power to said AC load throughout saidtransitioning.
 2. The maintenance bypass switch as recited in claim 1wherein said selector switch selectively couples and decouples bothphase and neutral conductors of said AC primary and DC backup powersources.
 3. The maintenance bypass switch as recited in claim 1 furthercomprising a status indicator coupled to said selector switch forindicating said mode of said selector switch.
 4. The maintenance bypassswitch as recited in claim 1 further comprising an alarm circuit coupledto said selector switch for transmitting a signal to an external alarmwhen said AC primary and DC backup power sources are disconnected fromsaid AC load.
 5. The maintenance bypass switch as recited in claim 1wherein said inverter/switch module comprises a static transfer switch.6. The maintenance bypass switch as recited in claim 1 wherein saidselector switch comprises a rotary cam switch.
 7. The maintenance bypassswitch as recited in claim 1 wherein said selector switch causes aninverter/switch module DC breaker to trip, thereby decoupling saidinverter from said DC backup power source.
 8. In a backup power systemcapable of deriving power from alternate AC primary and DC backup powersources to provide continuous AC power to an AC load and including aninverter/switch module having DC and AC inputs and an AC output, amethod of decoupling said AC primary and DC backup power sources fromsaid backup power system to allow maintenance to be performed thereon,comprising the steps of:transitioning a selector switch from a normalmode in which said selector switch couples said AC primary power sourceto said AC input of said inverter/switch module, said DC backup powersource to said DC input of said inverter/switch module and said ACoutput of said inverter/switch module to said AC load to a standbybypass mode in which said selector switch decouples an inverter in saidinverter/switch module from said DC backup power source when said ACprimary power source is functioning; further transitioning said selectorswitch from said standby bypass mode to a maintenance bypass mode, saidselector switch further coupling said AC primary power source directlyto said AC load; and causing said selector switch to enter saidmaintenance bypass mode in which said selector switch decouples said ACprimary power source from said AC input of said inverter/switch module,said DC backup power source from said DC input of said inverter/switchmodule and said AC output of said inverter/switch module to said ACload, said AC power provided continuously to said AC load throughoutsaid transitioning.
 9. The method as recited in claim 8 wherein saidsteps of transitioning, further transitioning and causing each comprisethe step of selectively coupling and decoupling both phase and neutralconductors of said AC primary and DC backup power sources.
 10. Themethod as recited in claim 8 further comprising the step of indicatingsaid mode of said selector switch with a status indicator coupled tosaid selector switch.
 11. The method as recited in claim 8 furthercomprising the step of transmitting a signal to an external alarm whensaid AC primary and DC backup power sources are disconnected from saidAC load.
 12. The method as recited in claim 8 wherein saidinverter/switch module comprises a static transfer switch, said step oftransitioning comprising the step of disabling a normal functioning ofsaid static transfer switch.
 13. The method as recited in claim 8wherein said selector switch comprises a rotary cam switch.
 14. Themethod as recited in claim 8 wherein said step of transitioningcomprises the step of causing an inverter/switch module DC breaker totrip, thereby decoupling said inverter from said DC backup power source.15. In a backup power system capable of deriving power from alternate ACprimary and DC backup power sources to provide continuous AC power to anAC load and including an inverter/switch module having DC and AC inputsand an AC output, a maintenance bypass switch having a DC breaker fordecoupling both phase and neutral conductors of said AC primary and DCbackup power sources from said backup power system to allow maintenanceto be performed thereon, comprising:a rotary cam selector switchconfigured to function in alternate normal, standby bypass andmaintenance bypass modes of operation, wherein:in said normal mode, saidselector switch couples said AC primary power source to said AC input ofsaid inverter/switch module, said DC backup power source to said DCinput of said inverter/switch module and said AC output of saidinverter/switch module to said AC load, in transitioning from saidnormal mode to said standby bypass mode, said selector switch causessaid DC breaker to trip, decoupling an inverter in said inverter/switchmodule from said DC backup power source when said AC primary powersource is functioning, in transitioning from said standby bypass mode tosaid maintenance bypass mode, said selector switch further couples saidAC primary power source directly to said AC load, and in saidmaintenance bypass mode, said selector switch decouples said AC primarypower source from said AC input of said inverter/switch module, said DCbackup power source from said DC input of said inverter/switch moduleand said AC output of said inverter/switch module to said AC load, saidmaintenance bypass switch providing said continuous AC power to said ACload throughout said transitioning.
 16. The maintenance bypass switch asrecited in claim 15 further comprising a status indicator coupled tosaid selector switch for indicating said mode of said selector switch.17. The maintenance bypass switch as recited in claim 15 furthercomprising an alarm circuit coupled to said selector switch fortransmitting a signal to an external alarm when said AC primary and DCbackup power sources are disconnected from said AC load.
 18. Themaintenance bypass switch as recited in claim 15 wherein saidinverter/switch module comprises a static transfer switch.
 19. Themaintenance bypass switch as recited in claim 15 wherein said selectorswitch causes an inverter/switch module DC breaker to trip, therebydecoupling said inverter from said DC backup power source.
 20. Themaintenance bypass switch as recited in claim 15 wherein, intransitioning said selector switch from said maintenance bypass mode tosaid normal mode, said DC breaker remains tripped, said DC breakercapable of being reset separately to recouple said inverter to said DCbackup power source.